The present invention relates to a novel fluorine-containing copolymer, a fluorine-containing resin composition which has excellent impermeability to liquid chemicals and a molded article using the same
In the field of producing semi-conductors, a large amount of liquid chemicals and water have been conventionally used in wet processes. A fluorine-containing resin having excellent chemical resistance, heat resistance and melt moldability is used for pipes transporting such liquid chemicals. Among fluorine-containing resins, a copolymer (PFA) of tetrafluoroethylene (TFE) and perfluoro(alkyl vinyl ether) (PAVE), particularly a copolymer of perfluoro(propyl vinyl ether) (PPVE), is excellent in chemical resistance, heat resistance, melt-moldability and stress cracking resistance, and thus preferably used for piping arrangements such as tubes and joints for transporting liquid chemicals.
However, these PFA piping arrangements have a problem of chemical liquid permeation in a small amount and needs some improvement. In current semiconductor production plants, there are some countermeasures such as covering the outside of a PFA tube with a pipe made of poly(vinyl chloride) (PVC) to make a double piping structure, or in the wet station area, exchanging PFA tubes regularly, wrapping them by a PVC film or further wiping the outside of a tube with a cloth.
These measures increase equipment and maintenance costs as might be expected, resulting in the increase of the cost for producing semiconductors.
As mentioned above, it is now practically difficult to solve the liquid chemical permeation problem of PFA in view of structure and maintenance, and therefore improvements are investigated with respect to its materials.
For example, there is a measure to use a fluorine-containing resin more impermeable to liquid chemicals, and among the fluorine-containing resins, it is effective to choose poly(chlorotrifluoroethylene) which is the most impermeable to liquid chemicals. However, this resin has a problem that it is poor in stress cracking resistance, moldability and heat resistance.
Furthermore, there is a way to increase crystallinity of PFA. It is effective to increase crystallinity since permeation of liquid chemical is generally observed at amorphous parts. As to PFA, it is possible to increase the crystallinity by decreasing the amount of perfluoro(alkyl vinyl ether) (PAVE). In this case, however, there arise defects that processability and crack resistance are decreased.
In addition, Japanese Unexamined Patent Publication No. 259216/1998 discloses the improvement of impermeability to liquid chemicals by modifying PFA using, as a third component, fluorovinyl ether which contains a reactive group such as a hydroxyl group. However, chemical resistance is decreased on the contrary since the terminal group of the modifying monomer is a reactive group represented by xe2x80x94CH2OH. Further, permeability to liquid chemicals are evaluated only in terms of weight increase after immersing the material in fuming sulfuric acid for four weeks at room temperature, and there is no substantial description of the permeation amount of liquid chemical in the publication.
On the other hand, Japanese Unexamined Patent Publication No. 116706/1999 discloses a process for irradiating a molded article of a fluorine-containing resin such as PFA or a copolymer (FEP) of tetrafluoroethylene and hexafluoropropylene with ionizing radiation under inert gas atmosphere and at temperature of at least the melting point to cross-link the polymers, thereby improving gas barrier property. However, special apparatus is required for such treatment, and this is not preferable from an economical point of view.
An object of the present invention is to provide a melt-moldable fluorine-containing copolymer which can provide, in an economically efficient manner, a molded article having excellent impermeability to liquid chemicals while retaining the excellent heat resistance, stress resistance and processability inherent in PFA-based fluororesins, a fluorine-containing resin composition and a molded article using the same.
As a result of intensive studies, it has been found that chemical impermeability can be improved without losing inherent properties of a resin by modifying a tetrafluoroethylene copolymer using a particular perfluoro(alkyl vinyl ether) and the present invention has been completed.
The present invention relates to a fluorine-containing copolymer comprising 90 to 99.4% by mole of a repeating unit derived from tetrafluoroethylene, 0.5 to 5% by mole of a repeating unit derived from at least one lower perfluoro(alkyl vinyl ether) represented by the formula (1):
CF2xe2x95x90CFxe2x80x94Oxe2x80x94Rf1xe2x80x83xe2x80x83(1)
in which Rf1 is a perfluoroalkyl group having 2 to 4 carbon atoms, and 0.1 to 5% by mole of a repeating unit derived from at least one higher perfluoro(vinyl ether) represented by the formula (2):
CF2xe2x95x90CFxe2x80x94Oxe2x80x94Rf2xe2x80x83xe2x80x83(2)
in which Rf2 is a perfluoroalkyl group having 5 to 10 carbon atoms or a perfluoro(alkoxy alkyl) group having 4 to 17 carbon atoms, wherein the fluorine-containing copolymer has a melt flow rate of 0.1 g/10 minutes to 100 g/10 minutes and a melting point of 290xc2x0 to 325xc2x0 C.
It is preferable that Rf2 in the higher fluoro(vinyl ether) represented by the formula (2) is a perfluoroalkoxyl group represented by the formula (3):
xe2x80x94(CF2C(CF3)FO)nxe2x80x94Rf3xe2x80x83xe2x80x83(3)
in which n is an integer of 1 to 4 and Rf3 is a perfluoroalkyl group having 1 to 5 carbon atoms.